Demons play an important role in the mythology of many cultures, from the asuras of Hinduism to the tzitzimimeh of the Aztecs. Their universal appearance in religions and cultures from all over the world perhaps speaks to a fundamental desire to understand the nature of evil and the motivation behind evil acts. The mythology of demons is quite complex, with many cultures having unique hierarchies and a number of famous named demons, such as Satan in Christian mythology.
In all cases, demons have supernatural powers, and many cultures have superstitious traditions which are supposed to repel them, ranging from leaving offerings to wearing particular objects to ward them off. Many cultures also have an assortment of friendly and evil demons, although it can sometimes be hard to tell the difference.
There are numerous types of demons. Some, for example, are merely playful earth or nature spirits who enjoy making mischief, while others are classified as tempters who lead people into sin. They may also work in opposition to a deity or deities, or they may assist witches and other evildoers. Some cultures classify them as human, meaning that they are the spirits of the dead, or nonhuman, and demons may possess people to do evil, or act independently.
The most benign sorts are things like imps, sprites, and poltergeists, largely considered to be obnoxious, but relatively harmless, in the cultures where they appear. Many cultures have some form of an imp to explain mysterious happenings, suggesting that these demons simply enjoy toying with people, but they aren't particularly malicious. These types were also features in Greek mythology, and in fact the word “demon” is derived from the Greek word for a nature spirit.
Many cultures have a tradition of demons which act as tempters to lead people from the path of righteousness. Incubi and succubi, for example, appear in seductive male and female forms, and Mara appeared to the Buddha to tempt him. By resisting such tempers, the faithful can demonstrate their commitment to their religion.
Demons may work in concert with people, as in the case of a witch's familiar, and they can also possess people, both voluntarily and involuntarily in many cultures. Demonic possession is a theme in many cultures, with their expulsion being a very profitable profession in some parts of the world. They may also possess inanimate objects according to some religious traditions, causing these objects to become cursed.
Christian demons like Satan work in opposition to God, and are classified as fallen angels. In Christian mythology, they are ranked, with Satan at their head. In Islam, they are known as jinn, and they can benign and friendly, or evil, in which case they are known as devils. Buddhist demons are primarily tempters, while Hindu ones work against the gods. These creatures are also used in many religious to explain natural phenomena, like eclipses, storms, and floods.